Oracle error message TNS: Unable to connect to destination

Error Code: 8(12203), 12203, ORA-12203
Error Message: TNS: Unable to connect to destination

Resolution:
This is an Error generated by the Oracle Client when the Oracle Server can not be reached.

This error can be caused by several items:

The Listener on the Oracle Server is not running. Check with an Oracle Administrator 
The Oracle Server is off line. Check with an Oracle Administrator 
The network connection to the Oracle server is not fully up and running. Check all connection points between the Notes server and the Oracle Server 
A firewall exists between the Notes Server and the Oracle Server, and is not configured to allow access to both the Oracle Server's IP address and Port 
The ADDRESS Parameter in the TNSNAMES.ORA file is incorrect. Use TNSPING (or TNSPING80 on Oracle 8.0.x) to check the connection to the Oracle Server. Example of a successful connection:

D:\ORANT\BIN>tnsping80 oracle

TNS Ping Utility for 32-bit Windows: Version 8.0.5.0.0 - Production on 23-APR-99
10:24:39

(c) Copyright 1997 Oracle Corporation. All rights reserved.

Attempting to contact (ADDRESS=(PROTOCOL=TCP)(HOST=DBSERVER)(PORT=1521))
OK (50 msec)

Hope useful for everybody to solve such problem.
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Yasser